Electronic devices having an audio function, such as mobile phones including smartphones, digital cameras, and wearable terminals, have become increasingly widespread. The housing of such an electronic device having an audio function encloses an audio part including a sound emitter such as a speaker and/or a sound receiver such as a microphone. The housing of the electronic device is typically provided with an opening positioned in register with such an audio part, and sound is transmitted through this opening between the outside of the electronic device and the audio part. For example, a microphone included in such an electronic device is typically a small-sized condenser microphone, which is enclosed in the housing of the electronic device in the form of a microphone unit including a package (housing) and a sound transducer enclosed in the package. Sound coming from the outside is directed to the sound transducer of the microphone through an opening formed as a sound inlet port in the housing and through an opening formed as a sound inlet port in the package. If a foreign matter such as dust comes in through the sound inlet ports, the foreign matter shakes in the vicinity of the sound transducer to cause noise. Entry of a foreign matter into the sound transducer also leads to failure of the microphone. Thus, in general, a sound-permeable membrane that prevents entry of foreign matters while permitting passage of sound is placed over the opening of at least one member selected from the housing and the package. The placement of the sound-permeable membrane also reduces noise induced by wind or breath. Possible examples of the foreign matters include water in addition to dust.
A porous sheet having air permeability is used as the sound-permeable membrane. JP 2008-199225 A states that a woven fabric or non-woven fabric of fibers formed of a resin such as nylon or polyethylene can be used as a sound-permeable membrane. JP 2007-81881 A states that a porous membrane of polytetrafluoroethylene (PTFE) can be used as a sound-permeable membrane. Given the properties of the porous PTFE membrane, this membrane is expected to prevent entry of foreign matters, including not only dust but also water. JP 2011-78089 A states that a non-porous film formed of a resin such as PTFE, polyester, polycarbonate, polyethylene, or polyimide can be used as a sound-permeable membrane, although JP 2011-78089 A does not disclose a porous sheet.